Concrete is a brittle material that exhibits a low tensile strength and fracture toughness, not because it is inherently weak in tension, but because it contains cracks that weaken it. The ease with which cracks can nucleate and propagate in concrete under tension makes it necessary to reinforce or toughen concrete to improve the life of a structure.
The method of reinforcing concrete by adding randomly distributed fibers throughout the volume of the matrix is known. The many different types of fiber materials used to reinforce concrete include steel, glass and organic fibers such as nylon, polypropylene, and polyethylene, as described in “Fibrous Reinforcements for Portland Cement” by S. Goldfein in “Modern Plastics” (April 1965, p. 156-159).
Depending on the size and concentration of the fibers, two types of reinforcement can be obtained in fiber reinforced concrete specimens. Very fine fibers dispersed in a concrete matrix at fractional volume percentages are effective in resisting the initiation and growth of small cracks that are caused by the shrinkage of the concrete upon hardening. Larger structural fibers, such as steel or glass, control the propagation of large cracks that cause ultimate failure.
For structural fibers to most effectively toughen a concrete specimen, it is important that: the fibers be added at a sufficient volume percentage, the fibers be well dispersed in the concrete matrix, the fibers be sufficiently well bonded to the concrete matrix, and the fibers be long enough to bridge a crack. To toughen a concrete matrix with randomly oriented fibers, it is beneficial for the fibers to have high tensile properties. For these reasons, very strong and stiff steel fibers dominate the structural concrete fiber market today. These fibers, either straight or crimped, have been shown to significantly improve the residual strength of concrete after a crack has formed. However, there are several limitations to using metal fibers, including cost and oxidation of the fibers over time.
Synthetic fibers have also been shown to provide toughening in concrete. However, one problem with using high modulus synthetic fibers in concrete is that high modulus fibers are typically very fine. Unfortunately, there are two major problems with using these fine fibers with high surface area as structural fibers in concrete. First fine fibers are difficult to disperse into concrete in a practical setting at concentrations higher than 0.1 volume percent. Because of the high aspect ratio of fine fibers, they tend to agglomerate in the concrete matrix, rather than uniformly distribute when added at loadings of greater than fractional volume percentages. The second problem with using the fine fibers at sufficiently high concentrations to structurally reinforce the concrete is that fine fibers are prohibitively expensive.
Despite the cost limitations, several efforts to improve the distribution of the fine fibers in concrete have been disclosed; which describe dispersing bundles of these fine fibers into the concrete matrix and after some time in the mixing process, the individual fibers are said to disperse from the bundle.
Patent (E.P. Pat No 0,488,577 B1) utilizes a wetting agent and (U.S. Pat. No. 5,993,537) discloses the use of an inorganic binding agent, both to bundle the fine fibers and break down in the cement mixing process and hence uniformly distribute the fine fibers. (U.S. Pat. No. 4,524,101) utilizes a compressive nip roll to deform the individual filaments and form a loosely adhering bundle of filaments. The bundle is dispersed into the concrete and the individual filaments are distributed as the bundle breaks apart during mixing with the concrete.
U.S. Pat. No. 6,197,423 B1 discloses a fiber that is passed between two nip rolls, thereby flattening and causing micro-diastrophism. When mixed in the concrete, some fraction of the fibers can fibrillate and separated from the large fiber, thereby providing small fibers that inhibit crack initiation.
Another form of bundling fine fibers for easier dispersion into concrete matrices is to fibrillate a thin film into a bundle or net of interconnected fibers. The addition of fibrillated plastic filaments to cement mortar or concrete has been disclosed in U.S. Pat. No. 3,591,395. Numerous patents have disclosed variations of fibrillated films that have been used to reinforce concrete (U.S. Pat. No. 3,591,395); (G.B. Pat. No. 2,034,243A); (G.B. Pat. No. 1,605,004); (E.P. Pat. No. 0,026,581); (U.S. Pat. No. 4,414,030); (U.S. Pat. No. 5,330,827); (U.S. Pat. No. 5,456,752); (U.S. Pat. No. 5,628,822); (G.B. No. 1,130,612). All of these fibrillated films take the form of discrete groups of interconnecting fibers. In each of these patents, the films are fibrillated using a rotating drum or roller having cutting elements such as needles or teeth in contact with the moving film. The result of each is a fibrillated film, made of interconnecting fibers that are proposed to break apart into individual fibers during mixing with the concrete.
A problem with the use of fibrillated films made of interconnecting fibers is that the individual fibers may not break apart during mixing with the cement and complete dispersal of the fibers is not achieved. To ensure that the fibers harvested from a film do indeed separate into individual fibers and disperse into the concrete matrix, U.S. Pat. No. 4,261,754 disclosed the benefit of fully fibrillating a film into individual fibers, prior to mixing into the concrete.
As with the fine fibers, the addition of large amounts of the larger fibers into concrete also causes balling and limits the distribution of fibers in the concrete matrix. Again methods were developed that allow for these fibers to be uniformly distributed into the concrete matrix (U.S. Pat. No. 3,716,386), (W.O. Pat. No. 00/49211), (U.S. Pat. No. 5,807,458), and (U.S. Pat. No. 5,985,449). One particularly useful method of dispersing up to 7 wt-% or higher of fibers into the concrete matrix is through the use of a low aspect ratio bundle-wrap that releases the fibers in the concrete matrix after sufficient time to allow each of the bundle to adequately disperse (U.S. Pat. No. 5,807,458).